A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The projection system used for projecting the patterned beam of radiation onto a target portion of the substrate is supported on a frame by a mount. The mount includes piezo-electric actuators and piezo-electric sensors, generally arranged in series with each other. The piezo-electric sensor measures the total force applied to the projection system and the detected force is used as part of a feedback loop for the piezo-electric actuator such that the forces on the projection system are minimized. By minimizing the forces on the projection system it remains substantially stationary. However, if the frame supporting the projection system and/or the substrate table and/or the support for the patterning device move or vibrate while the projection system remains stationary alignment errors could result.